1. Field of the Invention
The present invention relates to a method for detecting leaks in a cavity such as that between semiconductor wafers. More particularly, this invention relates to an electrical verification technique and device for detecting a leak within a cavity enclosing a micro-machined sensing structure.
2. Description of the Related Art
Many micro-electrical-mechanical systems (MEMS) devices such as yaw (angular rate) sensors, accelerometers and pressure sensors employ multiple substrates joined together into a functional stack. For instance, micro-sensor devices that are formed in silicon substrates (referred to herein as a device wafer) are often protected from moisture and particulates by bonding a cap (referred to as capping wafer) to the top surface of the sensor. When the bond is not complete and there is a leak path into the interior of the device stack, moisture and particulates can make their way through the void. Particulates can prevent proper movement of any movable portions of the sensor. Moisture can cause micro-machine surfaces of the sensor elements to permanently adhere to each other, or form ice crystal at low temperatures, thus preventing proper translation in response to mechanical stimuli. Further, materials common to packaging, including protective, stress relieving, or dielectric gels, can be wicked into the voids during the packaging of the sensor and interfere with sensor operation. This can render the sensor inoperable.
Various bonding materials and methods have been employed for the purpose of maximizing the strength and reliability of the bond. However, devices whose cavities are not hermetically sealed after the bonding operation have been unavoidably produced. Devices with inadequate seals are defective and need to be identified following the bonding operation. Current methods for identifying unsealed devices include visual inspections, which can be useful, but are expensive and compromised by the likelihood of human error. Other automated inspection techniques are also available, though each have limitations and are often expensive to implement in a large-scale assembly process.
For example, U.S. Pat. No. 6,074,891, assigned to the assignee of the present invention, the disclosure of which is explicitly incorporated by reference, discloses an electrical verification technique and semiconductor device that detects moisture within the cavity as an indication of whether the sensing structure is hermetically sealed within the cavity. The technique employs a bare, unpassivated PN junction diode in a semiconductor substrate. The reverse diode characteristics of the PN junction diode are then determined by causing a reverse current to flow through the diode. For this purpose, either a known voltage is applied across the diode and the reverse leakage current measured, or a known reverse current is forced through the diode and the voltage measured. The unpassivated junction diode exhibits measurable current/voltage instability, if sufficient moisture is present within the cavity, indicating that the cavity is not hermetically sealed.
Theoretically any device that is not hermetically sealed may be detected electronically if sufficient moisture is present within the cavity of the device. It was found that sufficient moisture normally requires water vapor of greater than about 50% relative humidity. At this level, the moisture would cause measurable instability in the diode. However, in high volume testing conditions, staging wafers in air of variable humidity occasionally allows leaky devices to escape.
In existing practices, to enhance the detection of the leaky devices, water is forced into any improperly sealed devices to assure an instable diode. However, if the leak is large, the water that is forced into the leaky device is free to drain out. Once the liquid is gone, the cavity may have an opportunity to dry out, and the diode may not be measurably instable. Thus an unsealed device is not detected.
Therefore, there is a need in a method and a device that assures the identification and containment of all unsealed devices.